1. Field of the Invention
The present invention relates to an image display device having a large screen.
2. Description of the Background Art
Examples of an image display device include a flat-panel television mounting a plasma display panel or a liquid crystal panel thereon, a rear transmission projection television, and the like. The rear transmission projection television projects an image on a screen disposed at the front surface of a main body in enlargement from an optical engine. As the optical engine is used a light source such as a high-pressure mercury-vapor lamp or a semiconductor laser housed inside of the optical engine.
On the back side of the image display device is disposed a light emitting unit for a high pressure current-carrying circuit or a high output semiconductor laser. Therefore, the back side of the image display device is covered with a back cover. An image display device may be configured such that the back cover is provided with an opening/closing detecting switch so as to prevent a service person from getting an electric shock or directly receiving a laser light beam when the service person opens the back cover for repair. Specifically, the image display device provided with the opening/closing detecting switch is configured to actuate the opening/closing detecting switch when the back cover is opened so as to cut off power supply to a drive circuit.
The size of a screen in the image display device has been increased in recent years. A display device having a size of 80 inch or more has been commercially available. The weight of the image display device has been increased with the increasing site of the screen, and therefore, the center of gravity has become relatively higher.
As a consequence, the image display device is likely to fall down when it receives external force or an earthquake occurs. Should the image display device fall down, there arise the following problems: one of the problems is that when a user touches the broken image display device, he or she gets an electric shock; and another problem is that in the rear transmission projection television, when the front screen is broken or detached, a strong laser light beam leaks from inside to outside.
In view of the above, in order to prevent the image display device from falling down, an instruction manual instructs to fix the image display device onto a wall or to a television stand via a string or a metal fitting before use. However, in the case where the user installs the image display device without fixing it, an accident of falling-down of the image display device often occurs.
In order to secure safety at the time of the falling-down of the image display device in the above-described case, there are devised the following techniques: far example, Japanese Patent Application Laid-Open No. 2000-287146 discloses the technique for detecting an impact upon inclining or falling-down by a detecting switch disposed in an image display device and then cutting off a power source (hereinafter also referred to as Conventional Art A). Conventional Art A prevents a failure in an image display unit.
Alternatively, Japanese Patent Application Laid-Open No. 2009-109650 discloses the technique for detecting falling-down of an image display device that is automatically started to be operated upon receipt of an emergency alarm signal (hereinafter also referred to as Conventional Art B). According to Conventional Art B, a drive power source in a display unit is controlled not to be turned on in the case of detection of the falling-down of the image display device. Moreover, according to Conventional Art B, the falling-down is detected by utilizing the mechanical movement amount of a detection lot housed inside of a strut for supporting a bottom in the image display device.
Or, Japanese Patent Application Laid-Open No. 2007-214751 discloses an image display device (i.e., a television) provided with a sensor for detecting a positional variation and an acceleration in the image display device, a microcomputer for calculating an output signal from the sensor so as to determine whether as main body falls down or is moved, and a battery. The image display device (i.e., the television) utilizes the technique for switching a power source from a commercial power source to the battery in response to a command output from the microcomputer at the time of the falling-down of the image display device (hereinafter also referred to as Conventional Art C).
However, a detecting circuit (i.e., the sensor) disclosed in Conventional Arts A and B is adapted to detect only one state, that is, the inclination (i.e., the falling-down) of the image display device (i.e., the television). The sensor disclosed in Conventional Art C also detects only one state, that is, the acceleration of the image display device (i.e., the television).
In other words, according to Conventional Arts A, B, and C, another detecting circuit (i.e., another sensor) need be provided for detecting states other than the falling-down state of the image display device. The states other than the falling-down state include, for example, a state in which a cover covering a portion supplied with electric power is detached.